Difference between int and Integer

问题:

What is the difference between int and Integer. Yes, one is primitive and another one is wrapper, what is the situation to use them correctly.

Also what is the difference between :

int i=0;
++i
and 
i++

回答1:

part 1

One example .. you can use Integer as the key of HashMap but you can't use int. Because an Object is needed.

So where you need an int value as an object there you need to use Integer class.

part 2

++i is pre increment i++ is post increment

for example

i = 0;
System.out.println(i++) //will print 0 then the i will be 1.

and

i = 0;
System.out.println(++i) // here i wil be incremented first then print 1.

回答2:

Integer is a wrapper class for int which is a primitive data type. Integer is used when int can't suffice. For example: In generics, the type of the generic class, method or variable cannot accept a primitive data type. In that case Integer comes to rescue.

List<int> list;   //Doesn't compiles
List<Integer> list;  // Compiles

Moreover Integer comes with a plethora of static methods, like toBinaryString, toHexString, numberOfLeadingZeros, etc. which can come in very handy.

回答3:

As already explained above An Integer is an object, whereas an int is a primitive. So you can have a null reference to an Integer and a Set or List of them. You can not do that with an int

I find this null reference very useful, when i have to store int values in database. I can store a null value when I use Integer. But cannot do so when I use int.

回答4:

An Integer is an object, whereas an int is a primitive. So you can have a null reference to an Integer and a Set or List of them. You can not do that with an int.

回答5:

A basic explanation is an int is a primitive data type and literally is only a value stored in memory. An Integer is a Java object that wraps an int in a Class with lots of nice/helpful methods that can be called to work with that backing int hidden inside. This is the same with most of the primitive data types, such as boolean and Boolean, char and Character, etc. This is refereed to as Boxing a primitive. Unboxing being the opposite, taking an Object and extracting the backing primative.

Here's an example of how one may use Integer to convert a String into an int (boxed to an Integer)

String someString = "10";
Integer intObj = Integer.parseInt(someString);
System.out.println(intObj.toString());

You'll find that some of the data types have more helpful methods than others. Check the JavaDoc's for each of the types you are interested in, there are a lot of goodies in there!